1. Field of the Invention
The present invention generally relates to a head-up display apparatus employing a fluorescent display tube capable of displaying various data of an automobile such as a vehicle velocity and an engine revolution speed. More specifically, the present invention is directed to a head-up display apparatus from which a heated filament image of the fluorescent display tube is completely eliminated while the display apparatus is turned off.
2. Description of the Related Art
A head-up display apparatus employing a fluorescent display tube described in the preamble is known from, for instance, Japanese KOKAI (DISCLOSURE) utility model application No. 50-73138 opened in 1975. Referring now to FIGS. 1 and 2, one conventional head-up display apparatus will be summarized. FIG. 1 illustrates the head-up display apparatus installed inside an automobile 1. Within the automobile 1, there is provided an instrument panel 2 which forms a front wall therein. A windowshield 3 is positioned above the instrument panel 2 at a predetermined inclined angle with respect to a vertical line of FIG. 1. A concave 4 is formed in the instrument panel 2, in which an information display source 5 is provided. As this information display source 5, a fluorescent display tube 6 is employed (see FIG. 2), which is disclosed in Japanese magazine "JIDOSHA GIJITSU" on pages 212 to 217, published in 1988, vol. 42. The fluorescent display tube 6 includes a filament 7, a grid 8, and an anode 9 with a known positional relationship. As shown in the circuit of FIG. 2, the filament 7 is connected via a power supply circuit 10 to a battery 11. In the power supply circuit 10, there are employed a constant voltage power source 12 and a filament power source 13.
On the other hand, to the anode 9 of the fluorescent display tube 6, a driver circuit 14 is connected, and also connected to a normally-ON switch 16 via a display control circuit 15. The display control circuit 15 includes a first waveform shaping circuit 17 for receiving an ON signal from the display switch 16, a latch circuit 18 connected to the first waveform shaping circuit 17, a second waveform shaping circuit 19 for accepting a vehicle speed pulse "P.sub.U ", a counter circuit 20, and a decoder circuit 21 series-connected to the second waveform shaping circuit 19 and counter circuit 20.
With the above-described circuit arrangement, when the switch 16 in turned ON (i.e., the first waveform shaping circuit 17 is grounded) for driving the automobile 1. While the latch circuit 18 is under off-condition, if the normally-ON switch 16 is turned ON (connected), the off-condition of the latch circuit 18 is inverted and this inverted condition is stored therein, and thus the latch circuit 18 energizes the driver circuit 14 so as to apply an anode voltage to the anode 9. A filament voltage is applied to the filament 7 from a filament power source 13 of the power supply circuit 10 so as to accelerate thermal electrons and project them to the anode 9. Under the above-described conditions, the vehicle speed pulse "P.sub.u " is entered via a second waveform shaping circuit 19, a counter circuit 20, and a decoder circuit 21 to the above-described driver circuit 14 while driving the vehicle. The fluorescent display tube 5 displays a present vehicle speed, and thus the displayed vehicle speed image is optically projected onto the windowshield 3. As a consequence, a car driver can recognize the present vehicle speed based upon the above-described image projected onto the windowshield 3 while observing a front forward driving view field with a small viewing movement. If the switch 16 is turned off with maintaining the ON-condition of the latch circuit 18, the on-condition of the latch circuit 18 is inverted into the off-condition, and the off-condition thereof is stored in the latch circuit 18, and therefore the driver circuit 14 is disenergized. As a result, the anode potential becomes zero and thus the vehicle speed image projected onto the window shield 3 will disappear.
In the above-described conventional head-up display apparatus, there are the following drawbacks, because the representation and also erase of the image projected onto the windowshield are controlled by switching the display switch 16 so as to control the anode voltage via the driver circuit 14. More specifically, even when the vehicle speed image is erased by turning off the switch 16, the filament current is continuously supplied to the filament 7 from the power supply circuit 10, whereby the heating condition of the filament 7 of the fluorescent display tube 5 is still maintained. As as result, when in particular, an indoor of a vehicle becomes dark such as in a tunnel, the heated image of the filament 7 is projected onto the windowshield 3, and therefore a car driver has to watch such a disturbance image while observing a front forward direction. This may cause driver's eyes to be tired.